Cryptosporidium parvum, an opportunistic infection of AIDS patients, has become well recognized as a causative agent of persistent and life-threatening diarrhea for these individuals. Additionally, C. parvum can cause severe diarrhea in healthy humans, calves and other economically important food animals. Since both immunocompetent and immunocompromised individuals are affected by this emerging pathogen, worldwide attention has focused on possible control mechanisms. Recently, the circumsporozoite-like antigen (CSL), an approximately 1,300-kDa glycoprotein, was determined to contain a sporozoite ligand for intestinal epithelial cells. Furthermore, it has been determined that CSL recognizes an 85-kDa receptor that is conserved among mucosal epithelial cells. The hypothesis of this proposal is that C. parvum sporozoites invade host cells by attaching to a conserved receptor on the surface of mucosal epithelial cells, making this invasion process susceptible to receptor-mediated blockade. Specific aims used to test this hypothesis will include molecularly characterizing the 85-kDa receptor (CSL-R), present on the surface of human intestinal epithelium, bound by CSL during C. parvum sporozoite invasion of the host cell. Monoclonal antibodies (mAbs) will be produced against CSL-R and whole Caco-2 cell lysate and screened for the ability to inhibit C. parvum invasion. Anti- CSL-R and anti-Caco-2 (recognizing additional receptors) mAbs will provide valuable tools for analyzing present and future C. parvum zoite/host cell interaction studies. Aim 3 will focus on mapping the inhibitory epitope (or binding motif) of CSL-R recognized by anti-CSL-R inhibitory mAbs (or CSL) and the subsequent generation of short synthetic peptides that mimic the receptor motif. These peptides will be analyzed for the ability to inhibit sporozoite invasion from a receptor-based approach. Such characterizations of a host cell receptor/sporozoite ligand interactions for C. parvum will be essential to targeted drug discovery, immunization, and other specific modalities for life cycle disruption. Ongoing molecular studies of parasite ligand/host receptor interactions may present exploitable events for novel approaches in the control of cryptosporidiosis, and given the supporting data - CSL/CSL-R seems to be the logical starting point.